The mononuclear phagocyte system is responsible for removing soluble immune complexes from serum, is involved in the induction of the immune response, and is an important part of the effector limb of cell- mediated immunity (CMI). In all of these functions, the IgG receptor on macrophages plays a key role as a recognition unit and as a mechanism for bringing cells together. The studies outlined in this proposal are designed to clarify many aspects of macrophage receptors as they relate both to normal function and to the involvement of these cells in events leading to tissue damage. The IgG receptor on human peripheral monocytes will be studied quantitatively, using cells from normals and from patients with autoimmune diseases. Also studies will be carried out examining lymphocyte-macrophage interactions, particularly the effects of lymphocyte factors on macrophage IgG receptors. These experiments will utilize cells from normals as well as cells from patients with autoimmune diseases. Also studies will be performed on patient sera to search for the presence of any serum factors which may inhibit or block lymphocyte-macrophage interactions. An additional objective is to characterize the chemical nature of the IgG receptor on macrophages, and to study the effects of steroids and estrogens on this receptor. These experiments will be carried out both in vivo and in vitro, using rabbits under various treatment programs, employing a quantitative assay for macrophage IgG receptors developed in this laboratory.